Principles of simple and countercurrent distillation

The reader is reminded that the word distillation can be translated as separation drop by drop (see Chap. 2). It can therefore be used as a collective term for processes in which mixtures of mutually soluble liquids can be separated by evajmration and condensation of the liquid, the condensed part becoming richer in the most volatile component. The word gives no indication of the technique adopted in the sejjarating process. The terms simple distillation and countercurrent distillation , however, define the inode of operation (Fig. 23). In a strictly physical sense distillation need not produce any separation we also speak of distillation when a pure liquid is eva-jiorated, the vapour is condensed and the condensate is removed. 

The exchange of material can be described by the following basic equation  

The exchange of material takes place by diffusion through the phase interface. It depends on the diffusion constant, D, the diffusion paths, b, concentration, x, and the phase interface area, Oed, per unit of length. 

The driving force results from the differences of the concentrations in the two phases of the system (y — ). At thermodynamic equilibrium they are saturated, the driving force becomes zero and the exchange of material taking place at non-equilibrium ceases. 

In simple distillation the molecules emerging from the evaporating surface move uniformly until they reach the condensing surface. In countercurrent distillation part of the condensed vapour, termed reflux , returns to the boiler, meeting in its passage the rising vapour. Provision is made for intimate contact between the liquid and 

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